1. Field of the Invention
The invention relates to a sintered material based on aluminum oxide, a sol-gel process for its production, and a process for its use as an abrasive, ceramic powder or molded article.
2. Background Art
Oxidic abrasives, such as, corundum or zirconia-alumina, usually are produced by melting the oxide or oxide mixture, allowing the melt to solidify, crushing it, and grading it. Drawbacks of the process are its high energy expense and the limited possibility of influencing the mechanical properties of the product, which essentially are determined by the composition and the cooling-off conditions.
Sintered materials based on aluminum oxide also are known which are suitable for use as an abrasive. The first of these sintered products, as they are described, for example, in U.S. Pat. No. 2,278,442, were produced from pure aluminum oxide or aluminum hydroxide or from bauxite by a fine grinding, optionally adding sintering auxiliary agents or glass phase makers, compacting and sintering or hot pressing. However, it was shown that the materials thus obtained also exhibit no optimal grinding properties, since their crystallite size is, for example, in the range of 5 to 20 microns, while substantially smaller crystallites are necessary to reach a maximum hardness and toughness. In West German OS 3,604,848, it was proposed to grind alumina with additives up to a particle size of less than 0.1 micron and then to subject it to a multistage heat and sintering treatment. Although the process starts from reasonably priced raw materials, the grinding process is very expensive and its feasibility on a large scale is questionable.
The problem of comminution is avoided by the so-called sol-gel processes, which, for example, are described in U.S. Pat. No. 3,144,827 and West German OS 3,219,607. The previously known embodiments of the sol-gel process generally require the addition of magnesium oxide or a precursor of it or another compound of a spinel-forming metal.
Another known possibility is the addition of up to 60 percent by weight (calculated as ZrO.sub.2 and in relation to the sintered product) of zirconium compounds. In this connection, the additives preferably are introduced in the form of soluble salts or alcoholates, which leads to a considerable amount of organic substance or inorganic acid components, such as, a nitrate, in the sols and gels formed. These compounds must be completely volatilized or decomposed in the heat treatment before the sintering. On the other hand it is desirable to bring in the additives in the smallest possible amount, and, if possible, as an oxide or a hydroxide, so that just water must be removed before the sintering.
Further, it is desirable to be able to influence the properties of the sintered product, especially its fracture toughness and hardness, in a comparatively simple way, without using numerous different additives.